To replace the F# side of things? F# is pretty unique as far as performance/language/tooling goes. It's outclassed in specific cases, but overall it's a great package.

The alternatives you listed aren't known for being able to write top-performance idiomatic code (I've got something _working_ in F#, but it's ugly non-idiotmatic code). The overhead of a GC is just too much to pay when doing linerate networking. Rust allows me to keep nice, high-level, idiomatic style, without paying any overhead. I can account for almost every byte.